1. Field of the Invention
This invention relates to analog storage. In particular, the invention relates to multilevel analog storage.
2. Description of Related Art
Analog storage has been popular recently due to advances in analog signal processing. Applications of analog storage include recording and playback of analog voice signals.
Traditional techniques record analog signals by sampling and storing a sampled analog voltage value on a charging element, e.g., a capacitor, which forms the gate of an electrically erasable programmable read only memory (EEPROM) or a flash memory device. The sampled analog voltage is stored in its entire normalized value voltage referenced to ground. In part, level shifting, companding, and other analog signal processing are performed on the normalized signal to faithfully record and playback the complete signal. Detection technology has limited the resolution to a maximum of about 48 dBm or 8 bits (for digitized samples) of dynamic range. This dynamic range is not sufficient for use in many high resolution applications.
Therefore, there is a need to have an efficient technique to increase the dynamic range for high resolution analog storage.
In one embodiment of the present invention, a technique is provided to store and retrieve analog signals.
A write circuit comprises a sampler, a first hold circuit, a second hold circuit, and a differential amplifier. The sampler captures an instantaneous value of the voltage present during a particular time period of an input signal. The first hold circuit holds the sample value after the sample period. The second hold circuit may either alternate with the first hold circuit to store a second new sample or may be transferred the sample from the first hold circuit. One method uses an analog switch or a transfer gate when the first held value is ready to be replaced by the next value. The differential creates a voltage which is the analog difference between the two held values.
A read circuit is similar to the write circuit except that the differential amplifier is replaced by a summing amplifier.